Method of normalizing implant strain readings to assess bone healing

ABSTRACT

A device for treating bone in a living body includes (a) comprises an implant configured for attachment to a bone; (b) a first sensor measuring a strain on a first portion of the implant, the first portion of the implant being configured to be mechanically coupled to a weakened portion of a bone when the implant is coupled to the bone in a target position in combination; and (c) a second sensor measuring strain in a non-weakened portion of the bone.

PRIORITY CLAIM

The present application is a Continuation Application of U.S. patentapplication Ser. No. 14/750,716 filed on Jun. 25, 2015; which is aContinuation Application of U.S. patent application Ser. No. 12/909,220filed on Oct. 21, 2010, now U.S. Pat. No. 10,441,210; which claims thepriority to U.S. Provisional Application Ser. No. 61/253,583 filed onOct. 21, 2009. The entire disclosures of these patent(s)/application(s)are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system and method for tracking theprogress of bone healing and, in particular, systems and methods thatcalculate a ratio of strain at multiple locations along an implantand/or a bone

BACKGROUND

Strain gages can be placed on orthopedic implants to track the progressof bone healing. Upon initial implantation, the implants are expected toexperience higher levels of strain which decrease during healing as thebone begins to share more of the load with the implant. Currently,however, implant strain values need to be assessed with a known loadapplied to the bone in order to evaluate bone healing.

SUMMARY OF THE INVENTION

The present invention is directed to a device for treating bone in aliving body, comprising an implant configured for attachment to a boneand a first sensor measuring a strain on a first portion of the implant,the first portion of the implant being configured to be mechanicallycoupled to a weakened portion of a bone when the implant is coupled tothe bone in a target position in combination with a second sensormeasuring strain in a non-weakened portion of the bone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a system according to a firstexemplary embodiment of the present invention;

FIG. 2 shows a perspective view of a system according to a secondexemplary embodiment of the present invention;

FIG. 3 shows a perspective view of a system according to a thirdexemplary embodiment of the present invention;

FIG. 4 shows a side view of a bone fixation element of the system ofFIG. 3; and

FIG. 5 shows a perspective view of a system according to a fourthexemplary embodiment of the present invention.

DETAILED DESCRIPTION

The present invention may be further understood with reference to thefollowing description and the appended drawings, wherein like elementsare referred to with the same reference numerals. The exemplaryembodiment of the present invention relate to a system and method fortracking the progress of bone healing. In particular, the exemplaryembodiments describe systems and methods that calculate a ratio ofstrain at multiple locations along an implant and/or a bone. Anexemplary embodiment of the system may include a first sensor on asurface of the implant adapted to be positioned at a location proximatea weakened portion of the bone. Strain on the implant at this locationwill be affected by the strength or stiffness of the weakened bone andthe load placed on the bone by the patient. A second sensor may beplaced on the implant at a location in which strain measured by thesecond sensor is affected only by the load placed on the bone such thatthe measured strain is substantially unchanged by the bone healingprocess. Thus, a ratio between the strains measured by the first andsecond sensors provides information corresponding to bone healing,regardless of the load on the bone. It will be understood by those ofskill in the art that although the exemplary embodiment specificallydescribe tracking the healing progress of a leg bone, the presentinvention may be used to track the progress of healing of any loadbearing bone. It will also be understood by those of skill in the artthat although the exemplary embodiments specifically show and describetwo sensors, the present invention may include additional sensors alongdifferent areas of the bone to determine ratios corresponding to thebone healing progress of the different areas. In addition, althoughexemplary embodiments show a bone plate, the present invention may beused with any other fixation element such as, for example, screws,intramedullary devices, external fixators, spine fixation implants andprosthetics.

As shown in FIG. 1, a system 100 according to a first exemplaryembodiment of the invention comprises an implant 102 (e.g., a boneplate) and first and second sensors 104, 106, respectively. The implant102 is configured for fixation over a target portion of a bone 108 to,for example, fix a fracture 110 or to support a weakened portion of thebone 108. The first and second sensors 104, 106 are mounted along asurface 114 of the implant 102 such that the first and second sensors104, 106 may be mechanically coupled to the bone 108. Although thesurface 114 is shown as facing away from the bone 108 when the implant102 is fixed to the bone 108 in a desired location, it will beunderstood by those of skill in the art that the sensors 104, 106 may bemounted along any surface of the implant 102. For example, the sensors104, 106 may also be mounted on a surface of the implant 102 facing thebone 108 or a surface on a side of the implant 102. The first and secondsensors 104, 106, respectively, are positioned on the implant 102 sothat, when the implant is in a desired position on the bone 108, thefirst sensor 104 is located over a site of the fracture 110 while thesecond sensor 106 is separated from the fracture 110 over a healthy(i.e., solid) portion 112 of the bone 108 to measure levels of strainand/or load on the implant 102, at these positions along the implant102. The second sensor 106 should he isolated between two screws lockedin a healthy portion 112 of the bone 108 to measure a load on the bone108.

The sensors 104, 106 in this embodiment may be passively powered MEMssensors that are used to measure strain and include an interface forwireless connection to a data collection device as would be understoodby those skilled in the art. In another embodiment, the sensors 104, 106may be powered chips that are connected to a printed circuit board(PCB). This permits strain on the implant 102 to be measured andtransmitted to the data collection device for further processing withoutphysically accessing the sensors 104, 106. It will be understood bythose of skill in the art that the strain measurements detected by thesensors 104, 106 are not required to represent actual strain values, butmay include any signal that changes based on changing strains of theirsubstrates. For example, the MEMS sensors 104, 106 may be RF devicesthat deform when a strain is placed thereon, resulting in a frequencyshift caused by a change in capacitance of the sensors 104, 106 suchthat the frequency shift corresponds to a change in strain. As would beunderstood by those skilled in the art, an external device may beemployed to wirelessly provide a signal to the sensors 104, 106. Changesin a returned signal may then be measured to determine a level of strainto which the sensor is subject. A ratio of the strain measured by thefirst sensor 104 to the strain measured by the second sensor 106 maythen be determined by a physician or other professional to track healingprogress. Alternatively, the ratio may be determined by a processingdevice that may also store the strain measurements and the determinedratios (e.g., in an internal memory or on an external storage device) sothat changes in the ratio may be reviewed to more fully understand theprogression of the healing over time.

It will be understood by those of skill in the art that when the bone108 is initially broken or fractured, strain on the implant 102 at thelocation of the fracture 110 will vary based on changing mechanicalproperties of the bone 108 during the healing process and the loadplaced on the bone 108 (e.g., the weight that the patient places on theleg) while the strain measured in the healthy portion 112 varies basedonly on the load placed on the bone 108. Thus, taking a ratio of thestrains measured by the two sensors 104, 106 normalizes the effects ofthe load on the sensors 104, 106 providing data corresponding to thestiffness of the bone 108 at the fracture site 110. The ratio of themeasurements from the first sensor 104 to the measurements from thesecond sensor 106 during the healing process should trend in adecreasing pattern over time, whereas a lack of healing would show norecognizable trend over time.

As shown in FIG. 2, a system 200 according to a second exemplaryembodiment of the invention is substantially similar to the system 100,including an implant 202 and at least two sensors 204, 206. However,rather than both sensors 204, 206 being positioned on the implant 202,the first sensor 204 is located on a surface 214 of the implant 202 in aposition corresponding to a fracture of a bone 208, while the secondsensor 206 is placed directly on a solid portion 212 of the bone 208,outside a perimeter of the implant 202. Thus, the first sensor 204measures strain on the implant 202 at a position corresponding to thesite of the fracture 210 while the second sensor 206 measures strain onthe solid portion 212 of the bone 208. Similarly to the system 100, aratio between the strains measured by the first and second sensors 204,206 is determined and tracked to study the progress of healing in thebone 208. As indicated above, the ratio of the strain measurements fromthe first sensor 204 to the strain measurements from the second sensor206 trend in a decreasing pattern as the bone 208 heals, whereas a lackof healing will show no recognizable trend over time.

As shown in FIGS. 3-4, a system 300 according to a third exemplaryembodiment of the invention is substantially similar to the system 200,comprising an implant 302 and at least two sensors 304, 306. Similarlyto the first sensor 204, the first sensor 304 is placed on a surface 314of the implant 302 in a location corresponding to a position of afracture 310 of a hone 308 (when the implant 302 is mounted on the bone308 in a desired position) to measure strain on the implant 302 at theposition of the fracture 310 while the second sensor 306 is placeddirectly on a solid portion 312 of the bone 308. However, rather thanbeing placed on an exterior surface of the bone 308, the second sensor306 is placed within the solid portion 312 via, for example, a bonefixation element 316 (e.g., screw).

The second sensor 306 may be attached adjacent to a proximal end 318 ofthe bone fixation element 316 such that when the bone fixation element316 is inserted into the solid portion 312 of the bone, the secondsensor 306 contacts a cortical wall of the bone 308. The second sensor306 may be printed or mounted around a portion of the bone fixationelement 316 to measure deformation of the bone 308 which is directlyrelated to strain on the bone 308. The ratio of the measurements fromthe first sensor 304 to those of the second sensor 306 may then bedetermined to track healing progress in the same manner described above.

As shown in FIG. 5, a system 400 according to a fourth exemplaryembodiment of the invention is substantially similar to the system 100,comprising an implant 402 and first and second sensors 404, 406,respectively, both of which are mounted on the implant 402. Similarly tothe first sensor 104, the first sensor 404 is located on the implant 402in a position which, when the implant 402 is in the desired position,corresponds to the location of a fracture 410 so that the first sensor404 measures strain on the implant 402 at a position corresponding tothe site of the fracture 410. The second sensor 406 is positioned on aportion 420 of the implant 402 having greater flexibility than theportion of the implant 402 on which the first sensor 404 is mounted. Forexample, the portion 420 may be made more flexible than other portionsof the implant 402 by reducing a width (i.e., an extent of the implant402 across a bone facing surface thereof in a direction perpendicular toa longitudinal axis of the implant 402) and/or a thickness of theportion 420 (i.e., a distance between the bone facing surface and asurface thereof which faces away from the bone) as compared to remainingportions of the implant 402. In a preferred embodiment, the flexibleportion 420 is adjacent to an end 422 of the implant 402 so that thesecond sensor 406 is separated from the fracture 410 by a distance greatenough to ensure that the underlying portion 412 of the bone 408 issolid.

The second sensor 406 on the flexible portion 420 of the implant 402 isfixed to the solid portion 412 of the bone 408 via, for example, lockingscrews inserted in holes 424 on opposing sides thereof. The secondsensor 406 measures strain on a portion of the implant 402 correspondingto the solid portion 412 of the hone 408 so that measurements from thesecond sensor 406 may be used to normalize measurements from the firstsensor. Similarly to the placement of a sensor directly in or on a hone,as described in conjunction with systems 200 and 300, placing the secondsensor 406 on a more flexible portion 420 of the implant 402 between twolocked screws permits a more accurate measurement of the strain on theunderlying solid portion 412 of the bone 408, as compared to the resultsfrom placing the second sensor 406 on a stiffer portion of the implant402. The ratio of the measurements from the first sensor 404 to themeasurements from the second sensor 406 during the healing processshould trend in a pattern indicating an increasing stiffness of the bone408 over time, whereas a lack of healing should show no recognizabletrend over time.

It will be understood by those of skill in the art that other mechanismsmay be employed for normalizing measurements of strain on a portion ofan implant which, when mounted on a bone in a target location,corresponds to a position of a fracture or other weakened portion ofthat bone. For example, the patient may be provided with load sensors onwhich to push or stand with the affected limb such that a loadmeasurement may be taken simultaneously with a strain measurement of thesensor on the implant. Alternatively, the patient may be provided with asensor (e.g., placed in the sole of a shoe) to measure the load placedon the affected leg, if the affected bone is the femur or tibia.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the structure and themethodology of the present invention, without departing from the spiritor the scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided that they come within the scope of the appended claims andtheir equivalents.

1-20. (canceled)
 21. A system for treating a bone, comprising: animplant configured for attachment to an exterior of a bone; a firstsensor positioned to measure a first property of a first portion of thebone adjacent to the first sensor; a second sensor positioned to measurea second property of a second portion of the bone adjacent to the secondsensor; and a processing device determining a ratio between the firstand second measured properties to determine a degree of healing of thefirst portion of the bone.
 22. The system of claim 21, wherein the firstproperty correlates to a first strain measurement and the secondproperty correlates to a second strain measurement.
 23. The system ofclaim 22, wherein the first sensor is positioned on a first portion ofthe implant adjacent to the first portion of the bone.
 24. The system ofclaim 23, wherein the second sensor is positioned on a second portion ofthe implant adjacent to the second portion of the bone.
 25. The systemof claim 24, wherein first portion of the implant has a first bendingstiffness and the second portion of the implant has a second bendingstiffness less than the first bending stiffness.
 26. The system of claim23, wherein the second sensor is positioned directly on the secondportion of the bone outside a perimeter of the implant.
 27. The systemof claim 22, further comprising: a bone fixation element that isinserted into a portion of the implant adjacent to the second portion ofthe bone.
 28. The system of claim 27, wherein the second sensor ismounted to the bone fixation element so that the second sensor contactsa cortical wall when the bone fixation element is inserted into thesecond portion of the bone.
 29. The system of claim 22, wherein at leastone of the first and second sensors is a powered MEMs sensor or apowered chip connected to a printed circuit board, the at least one ofthe first and second sensors having an interface for wireless connectionto the processing device.
 30. The system of claim 29, wherein theprocessing device includes a memory storing the first and secondmeasured properties.
 31. The system of claim 30, where the memory storesprevious measurements so that current measurements may be compared tothe previous measurements to track the healing of the first portion ofthe bone.
 32. The system of claim 21, wherein the implant is a plateconfigured to be coupled to a bone overlying a fracture site.
 33. Asystem for treating a bone, comprising: an implant configured forattachment to an exterior of a bone; a first sensor positioned on afirst portion of the implant to measure a first property correlating toa strain of a first portion of the bone adjacent to the first portion ofthe implant; and a second sensor positioned to measure a second propertycorrelating to a strain of a second portion of the bone adjacent to thesecond sensor.
 34. The system of claim 33, wherein the second sensor ispositioned on a second portion of the implant.
 35. The system of claim33, wherein the second sensor is positioned either directly on thesecond portion of the bone or on a bone fixation element insertedthrough the second portion of the bone.
 36. A method for treating abone, comprising: attaching an implant to an exterior of a bone having afirst portion and a second portion; measuring a first property of afirst portion of the bone via a first sensor positioned adjacent to thefirst portion of the bone; measuring a second property of a secondportion of the bone via a second sensor positioned adjacent to thesecond portion of the bone; and determining, via a processing device, aratio between the first and second measured properties to determine adegree of healing of the first portion of the bone.
 37. The method ofclaim 36, wherein the first property correlates to a first strainmeasurement and the second property correlates to a second strainmeasurement.
 38. The method of claim 37, further comprising: positioningthe first sensor on a first portion of the implant adjacent to the firstportion of the bone; and positioning the second sensor on a secondportion or the implant adjacent to the second portion of the bone. 39.The method of claim 38, wherein first portion of the implant has a firstbending stiffness and the second portion of the implant has a secondbending stiffness less than the first bending stiffness.
 40. The methodof claim 36, further comprising: inserting a bone fixation element intoa portion of the implant adjacent to the second portion of the bone.